This invention relates to engine turbochargers and particularly to a novel ball bearing mounting of a high thrust turbocharger rotor.
A turbocharger for a medium speed diesel engine, adaptable for use in railway road locomotives and other applications, has a rotor with a radial flow compressor wheel or impeller and an axial flow turbine wheel or turbine, unlike typical automotive turbochargers. The wheels are carried at opposite ends of a connecting shaft supported at two spaced bearing locations with the wheels overhung. This configuration is known as a flexible rotor, since it will operate above its first, and possibly second, critical speeds. It can therefore be subject to rotor dynamic conditions such as whirl and synchronous vibration.
High thrust loads are created by the difference in air pressures across the turbine and compressor wheels. These loads can be quite large due to the relatively large radial area of the wheels. The net thrust loads on the wheels are in the same direction, creating a high overall thrust on the rotor. The radial load due to the static weight of the rotor is comparatively small.
Turbocharger design can include the use of sealing devices at the rim of turbine wheel to help control pressure on the face of the turbine wheel inboard of the blades. This is feasible because the high temperature turbine end materials have more closely matched thermal expansion coefficients than the aluminum wheel and ferrous housing materials typical of the compressor end of the turbocharger. Thus, at the turbine end, a reasonable range of clearances can be obtained.
On the upstream end, the aim is to keep the flowpath pressure off the face of the turbine wheel. This pressure pushes in the same direction as the thrust on the compressor wheel. On the downstream end, if the face could be pressurized it would help to reduce the compressor wheel thrust effect by pushing the other way. In practice, this is difficult, because the seal must be made very tight or else an extremely high flow of pressurized air is required, only to be directly exhausted out of the turbocharger without being used to do any work.
Diesel locomotive engines, and turbochargers, may operate over an extremely large range of conditions, from minus 40 degrees at startup to the high temperatures and high turbine speeds experienced in a high altitude tunnel. With aluminum compressor wheels chosen for low inertia and quick response, their rotating and static thermal coefficients are poorly matched to the housing so that sealing the back face of the compressor wheel is not a currently practical option. Since the compressor pressure ratio is considerably higher than that of the turbine, a higher pressure acts over an area about equal to that of the turbine.
Even with the use of seals where practical, and more so without them, the high thrust loads acting on the rotor, as well as the potential for whirl and vibration, have made hydrodynamic fluid film bearings the universal choice for turbochargers of this type as compared to the common use of ball bearings in automobile engine turbochargers. Hydrodynamic fluid film bearings feature high load capacity, variable stiffness, essentially infinite life if the fluid film is maintained, and allow large shaft diameter for better stiffness and lower vibration. However, they require high oil flow and cause high power losses, which reduce overall efficiency.
Ball bearings require much lower oil flow and operate with lower power loss for improved efficiency as well as more consistent stiffness over the operating range. However, they have lower thrust load capacity, have finite operating life due to metal fatigue of the moving parts, and must be limited in diameter so that high rotating speeds do not put excessive centrifugal loads on the balls. As a result, ball bearings are not known to have been successfully applied to turbochargers of the type described as used in railroad engines and other applications.
The present invention provides a turbocharger, adapted for use in railroad locomotive engines and other applications, combined with a ball bearing rotor mounting capable of accepting both radial support loads and axial thrust loads applied to the rotor of a railroad engine turbocharger.
In a preferred embodiment, the turbocharger includes a housing carrying a rotor having an axial flow turbine wheel and a radial flow compressor wheel. The wheels are supported at opposite ends of a shaft carried in the housing on oil lubricated first and second bearings spaced axially adjacent to compressor and turbine ends respectively of the shaft. The arrangement provides an overhung rotor mounting with axial thrust loading normally applied to the shaft from both wheels in the same direction from the turbine toward the compressor.
In the improved assembly, the first bearing includes at least one hybrid ceramic ball bearing mounted to accept both radial and axial loads acting on the shaft at the compressor end. The first bearing is mounted on a reduced diameter portion of the shaft, providing reduced bearing diameter to acceptably limit centrifugal loading of ceramic balls in the bearing against a surrounding bearing race.
Additional features may include a first bearing having dual rows of ceramic ball bearings mounted to share all axial thrust loads on the shaft. The second bearing may also be a ball bearing and, optionally, a hybrid ceramic thrust bearing on a reduced diameter shaft portion to limit centrifugal loading of the balls in the bearing. Lubrication of the bearings is preferably by direct impingement on the inner race to minimize oil churning causing heating and power loss. The shaft between the bearings preferably has a greater diameter than at the bearing locations to maintain adequate bending stiffness in the overhung rotor. The second bearing may be made slidable in the housing to direct all thrust loads to the dual row first bearing. A squeeze film damper may carry the second bearing to minimize whirl at the turbine end of the rotor. The shaft may be separate from the compressor and turbine wheels and include a yieldable fastener, such as a stud or bolt extending through the compressor wheel and the shaft to engage the turbine wheel and maintain a relatively constant clamping load on the rotor.